Due to various merits of organic light-emitting devices (OLEDs), such as high efficiency, wide viewing angles, fast response, potentially low cost etc., OLED technologies have become an important next-generation display technology since Dr. Tang and VanSlyke reported the first efficient and practical OLED in 1987. Due to increasing efficiencies, the OLED is also becoming practical for lighting applications. No matter display or lighting applications, external quantum efficiencies (EQEs) of OLEDs are essential. EQEs of OLEDs are determined by internal quantum efficiencies (IQEs) and the optical out-coupling efficiencies. Through appropriate combinations of materials for electrodes, carrier-transport layers (e.g., hole-transport layers-HTL, electron-transport layers-ETL), emission layers (EML), and their stacking, internal quantum efficiencies can reach nearly 100%. However, in typical OLED structures, optical out-coupling efficiencies of OLEDs are still limited.
Currently typical OLEDs are fabricated on a substrate. According to the direction of the light emission relative to the substrate, OLEDs can be classified into bottom-emitting OLEDs or top-emitting OLEDs. Bottom-emitting OLEDs 1 emit through the transparent or semi-transparent substrate 10 as FIG. 1, while top-emitting OLEDs 2 emit opposite the substrate direction as FIG. 2.
Please refer to FIG. 1, the bottom-emitting OLEDs 1 are typically composed of a single or multiple organic material layers 12 stacking and sandwiched between top reflective electrode 13 and bottom (semi-)transparent electrode 11. Through appropriate combinations of materials for electrodes, carrier-transport layers (e.g., hole-transport layers-HTL, electron-transport layers-ETL), emission layers (EML), and their stacking, internal quantum efficiencies can reach nearly 100%. However, in typical bottom-emitting OLED 1 structures, for example, glass or plastic substrate/transparent electrode such as ITO/organic layers/reflective electrode such as Al, due to higher refractive indices n of the organic layers (typically n≥1.7) and transparent electrodes (typically n≥1.8) than those of substrates (e.g., n˜1.4-1.5 for glass substrates), a significant portion of internally generated light with larger angles will be confined in the device by total internal reflection at the electrode-substrate interface and cannot enter the substrate for out-coupling into air. For the light entering the substrate 10, due to higher refractive indices of transparent substrates (e.g., n˜1.4-1.5 for glass substrates) than that of air, again a significant portion of light with larger angles will be confined in the substrate by total internal reflection at the substrate-air interface and cannot be out-coupled into air. As such, in typical bottom-emitting OLED structures, optical out-coupling efficiencies are generally limited to only 20-25%.
On the other hand, the typical top-emitting OLEDs 2 have the structure of substrate 20 such as glass or plastic/bottom reflective electrode such as metal 21/organic layer(s) 22/top (semi-)transparent electrode 23 such as ITO, thin metal, as shown in FIG. 2. In some cases, the top (semi-)transparent electrode may be further over-coated with transparent passivation or capping layer. Due to higher refractive indices of organic layers (typically n≥1.7), transparent electrodes (typically n≥1.8), and even transparent passivation or capping layers than that of air, a significant portion of internally generated light with larger angles will be confined in the device by total internal reflection at the device-air interface and cannot be out-coupled to air as shown in FIG. 2. Therefore, in typical top-emitting OLED structures, optical out-coupling efficiencies are generally also limited.
Therefore, to achieve high-efficiency, power-saving OLED displays or lighting, the optical out-coupling efficiencies have to be effectively raised by out-coupling otherwise trapped OLED internal light. This invention thus aims to provide OLED device structures that can effectively enhance optical out-coupling efficiencies of OLEDs.